Pipeline for transport of gas

ABSTRACT

Pipeline for transportation of gas, for which the supplying pressure for the gas out through an outlet shall not exceed a threshold value, where the pipeline comprises a safety unit with a pressure transmitter and a temperature transmitter arranged in connection to the pipeline close to an inlet of the pipeline, distinguished in that the safety unit comprises a device for measuring the flow rate and the safety unit is either used for maintaining an increased inlet pressure to the pipeline and/or in order to use a pipeline with a reduced inner diameter, within a given safety level against leakage and excessive pressure.

FIELD OF THE INVENTION

The current invention relates to pipelines for transportation of gas. More precisely, the invention relates to gas pipelines that shall be operated within a given safety level against excessive pressure and gas leakage from the pipeline, even though the pipeline is connected to a pressure source that may over-pressurized the pipeline in relation to the pipeline's design code.

BACKGROUND AND PRIOR ART OF THE INVENTION

The authorities instruct operators of gas pipelines to operate said lines so that the risk of a gas leakage during a one-year period lies within a given safety level. Said authorities required instructions will vary from country to country, but will in all cases impose instructions as to how a gas pipeline shall operate. A typical safety level is that the yearly probability of a leakage and to exceed a given pressure within a margin of 2 bar, shall be less than 1×10⁻³, total for all sources for high pressure and all incidents, which may occur during the operation of the pipeline. In order to comply with said instructions, a safety device or safety unit is arranged in connection with the pipeline. The components, which may be used in connection with said safety device, must be approved by the authorities to have a reliability that meet said safety level. A crucial consideration in connection with the current invention is that the gas pipeline and all of the equipment determining the safety, must be within a given total safety level, why it is not relevant to use methods or components that are not approved for said safety level. There are a number of process control systems and leakage detection systems, which are not useful in relation with a safety device in connection to a gas pipeline.

Prior known safety devices for gas pipelines make use of pressure and temperature measurements. There are no previously known safety devices, which also make use of the measuring of the flow rate at the inlet of the gas pipeline. The gas' compressibility does not provide an immediate correlation between the flow rate, pressure and temperature in to the pipeline in relation to what is transmitted out of the pipeline. This is why safety devices suitable for a liquid conducting pipeline is not functional for a gas conducting pipeline.

In a typical instance, the gas pipeline shall be connected to an existing pipeline system, and the supplying pressure for the gas from the pipeline through an outlet must not exceed a given threshold value, decided by requirements given for the receiving pipeline system. The connecting of said pipeline and the pipeline system is typically carried out with a so-called hot tap connection. There is a need for a gas pipeline, which is particularly suitable for connecting to an existing pipeline system in accordance with the requirements as mentioned above.

SUMMARY OF THE INVENTION

The above mentioned need is met by the invention providing a pipeline for transportation of gas, for which the supplying pressure for the gas out through an outlet shall not exceed a threshold value, where the pipeline comprises a safety unit with a pressure transmitter and a temperature transmitter arranged in connection to the pipeline close to an inlet of the pipeline, distinguished in that the safety unit comprises a device for measuring the flow rate and the safety unit is either used for maintaining an increased inlet pressure to the pipeline and/or in order to use a pipeline with a reduced inner diameter, within a given safety level against leakage and excessive pressure.

The term a given safety level against leakage and excessive pressure refers to the supplying pressure for gas transmitting out from the pipeline through an outlet shall not exceed a threshold value, and that there must be no leakage from the pipeline within a given probability as defined by the safety level. In a preferred operational form of the invention, the safety level is such that the yearly probability for exceeding a given supplying pressure at the outlet, within a margin typically of 2 bar, taking into account all pressure sources, incidents and leakage from the pipeline itself, shall be less than 1×10⁻³, more preferable less than 1×10⁻⁵.

In a preferred operational form the arrangement for measuring the flow rate is a venturi tube, placed inside or close to an inlet of the pipeline. In a preferred operational form, the safety device is controllable connected to at least two emergency shutdown valves in serial within the pipeline, close to or inside the inlet of the pipeline. The safety device is operated in accordance with an algorithm, which calculates the outlet pressure from the pipeline, solely based on measurements of the pressure, temperature and flow rate inside or close to the inlet of the pipeline.

With the current invention achievement is made in that the existing gas pipeline can increase its capacity or that a new pipeline can be constructed with less inner diameter than previously, when compared with previous gas pipelines with a safety device arranged. This has a significant technical and economical advantage for the operator of gas pipeline and operators of related field and pipeline systems.

DETAILED DESCRIPTION

The device for measuring the flow rate may be approved for fiscal measuring, if it is verified reliable enough, in addition to being precise and reliable in relation to the determined safety level. There are ultra sound based fiscal flow rate meters for gas, there are coriolis meters usable for the same purpose and there are differential pressure meters based on a narrow part of the gas pipeline, such as with a venturi tube. It is only devices with venturi tubes which at the moment are within the safety level needed to be observed in the North Sea. The pressure transmitter, temperature transmitter or flow rate transmitter in connection with the venture tube is connected to the safety device, as is the pressure transmitter in connection with the gas pipeline, which is also connected to the safety device. The flow rate may be given directly from the venturi tube or determined by the safety device. The safety device is connected to at least two, preferably at least three emergency shutdown valves in series inside the gas pipeline, in or close to its inlet. Furthermore, the safety device is advantageously connected to a gas turbine or another source for pressure to the gas pipeline, so that a set point for said source is adjustable by means of signals from the safety device. By means of a suitable algorithm the safety device considers the transient behaviour of the gas inside the gas pipeline. It is therefore possible to maintain the given safety level without the need for a comprehensive instrumental system with measurements at the inlet of the gas pipeline and along the gas pipeline itself. This in itself saves a significant cost. All the elements inside the gas pipeline, the safety device and the connecting devices, may in itself be known, but it is the combination of these in a safety device for the operation of the gas pipeline within a given safety level, which provides a surprising technical effect. It is possible to increase the capacity of the existing pipeline since a higher inlet pressure for the gas may be used. It is possible to reduce the diameter for new gas pipelines, which will result into major savings on investment costs. It is possible to have a combination of some increased inlet pressure and some reduced inner diameter inside the gas pipeline, within the total safety level.

The algorithm which the safety device uses may, as previously mentioned, be known. The algorithm however, must be adjusted to the relevant parameters, including the parameters for the gas pipeline in each case. Flow programs, such as OLGA and HySYS should be used to simulate the algorithm functionality during different operational situations, including blocking by for instance hydrates inside the gas pipeline, and substantial increase of the flow rate, to verify that the maximum permitted pressure inside the pipeline in relation to the design code and the maximum permitted outlet pressure from the pipeline are not exceeded. The algorithm must be verified so that unnecessary shutdown from the safety device is avoided, at the same time as the shutdown taking place should not result in excessive pressure inside the pipeline in relation to neither the design code or the outlet pressure within the given safety level. The algorithm is typically built around polynomial expressions for the flow rate, pressure and temperature within given permanent dimension and with the adjustment of the parameters over time within given limits.

A description of the safety systems and safety devices relevant to the current invention can be found in the document DnV OS-F101. The existing safety systems are based on redundant downstream pressure measurements and triple redundancy for telemetry, pipelines with full design pressure over the full length or a large pipeline diameter to avoid that the exporting pressure is higher than acceptable for the existing pipeline systems downstream.

The existing pipeline systems is becoming more and more complex with constantly new branch lines being connected. Complex telemetry and complex reliability assessments prevent further development of the transport network in the form of pipeline systems. The current invention introduces a reliable flow rate measuring together with an existing safety system. This provides several advantages, whereas it is mentioned that the pressure at a weak point in a gas pipeline, is accurately as possible estimated by the safety system instead of measuring the said pressure. Furthermore, underwater pressure measurement by the connecting point is not necessary. Moreover, comprehensive telemetry arrangements are avoided, which is also cost effective. For the existing transporting system, the export pressure inside the pipeline can be increased to above the maximum pressure permitted in the fitted downstream pipeline system and thereby increase the export of the gas through the pipeline. For new systems the pipeline's inner diameter may be reduced, which reduces significantly the need for investments.

EXAMPLE

A gas pipeline with a length of 130 km is planned from the semi-submerged platform Gjøa to the pipeline system FLAGS. Connecting to FLAGS will be through a hot-tap together with the pipeline Tampen Link. The gas is then sent to St. Fergus in Great Britain for further processing and delivery to customers. By using a pipeline for gas transport with a safety device in accordance to the current invention the inner flow cross section of the pipeline may be reduced by 20%, for example from 720 millimetre to 660 millimetre in inner diameter. This will result in a saving on the pipeline itself of NOK 95 mill. Provided that this is operational sensible for Gjøa and adjoining sectors, then the inner pipeline diameter of 720 millimetres alternatively may be maintained whilst the flow rate through the pipeline is increased, whereas the exported quantity of gas may be increased by 15%. Alternatively, the inner diameter may be slightly reduced, whilst the exported quantity of gas is slightly increased within the given safety level.

The technical effect of the invention will vary a great deal and must be calculated for each case. Should the value of increased gas export and/or reduced pipeline diameters exceed the investment in an arrangement of a pipeline according to the present invention, then the invention represents an economical advantage. 

1-6. (canceled)
 7. Pipeline for transportation of gas, for which the delivery pressure for the gas out through an outlet shall not exceed a threshold value, where the pipeline comprises a safety unit with a pressure transmitter and a temperature transmitter arranged in connection to the pipeline close to an inlet of the pipeline, wherein the safety unit comprises a device for measuring the flow rate, the safety unit is connected to at least two emergency shutdown valves in series in or close to the pipeline's inlet, such that the input measurable parameters consist of pressure, temperature and flow rate by the inlet of the pipeline, and the output parameters comprises signal to isolate the source supplying gas to the pipeline, in the form of control signals for said two emergency shutdown valves arranged in series in or close to the inlet of the pipeline, and the safety unit can be used for maintaining an increased inlet pressure to the pipeline and/or in order to use a pipeline with a reduced inner diameter, within a given safety level against excessive pressure, said safety level is chosen so that during the course of one year, the probability for excessive pressure in or out of the pipeline within a margin of 2 bar, including all possible incidents and causes of high pressure, is maximum 1×10⁻³.
 8. The pipeline according to claim 7, wherein said safety level is chosen so that during the course of one year, the probability for excessive pressure in or out of the pipeline within a margin of 2 bar, or leakage, including all possible incidents and causes of high pressure, is maximum 1×10⁻⁵.
 9. The pipeline according to claim 7, wherein the device for measuring the flow rate is in the form of a venturi tube.
 10. The pipeline according to claim 7, wherein the safety unit is operated in accordance with an algorithm which considers the transient behaviour of the gas.
 11. The pipeline according to claim 7, wherein the input measurable parameters consist of pressure, temperature and flow rate by the inlet of the pipeline, and the output parameters consist of set point to a gas compressor delivering gas to the pipeline and control signals for three emergency shutdown valves arranged in series in or close to the inlet of the pipeline. 